The present invention relates to a liquid crystal display device comprising a liquid crystal cell having a pair of substrates each provided with transparent electrodes and a liquid crystal layer positioned between the substrates, a pair of polarizers disposed on opposite sides of the cell and a birefringent compensation plate disposed between the cell and at least one of the polarizers, being excellent in viewing angle compensation and color compensation and easy in manufacturing and handling.
Display modes of liquid crystal display devices so far mainly used are operated in a Twisted Nematic (hereinafter referred to as "TN") type, and have a structure in which liquid crystal molecules are twisted about 90.degree. between upper and lower substrates, and utilize a rotation of a plane of polarization by a liquid crystal and an elimination of effect thereof upon a voltage application. Since these modes are black and white display, there are advantages that they have excellent shutter effects and their multicolor display can relatively easily be prepared by placing color filters on each of picture elements. On the other hand, as a threshold characteristic of voltage-transmittance relationship is poor, it involves drawbacks, such as, a difficulty on a time division multiplex drive, low contract in a wide display or a narrow viewing angle.
In view of the above, for improving a steepness of the voltage-transmittance relationship, a birefringent type liquid crystal display device in a broad meaning, such as Super Twisted Nematic (hereinafter referred to as "STN") mode or a Super Twisted Birefringent Effect (hereinafter referred to as "SBE") mode utilizing a birefringent effect of liquid crystal by increasing a twisted angle of the liquid crystal molecules and shifting an axis of polarization of the polarizer and an aligned direction of the liquid crystal molecules was developed, and the drawbacks was overcome. While, as an effective refractive index anisotropy, .DELTA.n', and an optical path length, d; of the liquid crystal molecules change depending on a direction of the viewing angel, it has disadvantages that a back ground color and a contrast varies depending on a viewing angle. These disadvantages are the same with other birefringent modes than STN, such as Electrically Controlled Birefringence (ECB), Deformation of vertically Aligned Phase (DAP), Hybrid Aligned Nematic (HAN) and TN.
In case of STN, there is an additional disadvantage, that is, the background color is not achromatic because of a birefringent color. For reducing this color, it has been developed a two layer system in which two cell having twisted angle of the liquid crystal layers therein opposite to each other are laminated into a compensation cell and one of the cells is used fro driving while the other is used as a compensation plate. However, as a thickness and weight of the device are increased and the productivity is low, there exist problems of inconvenient handling and high production cost.
Although the above problems can be removed by using a compensation plate made of birefringent polymer films, as a viewing angle dependence of birefringence of the plate is large analogous to birefringent type liquid crystal display device, it has a disadvantage of narrow viewing angle.
Polymer films generally used for the conventional compensation plate are made of polymers such as polyvinyl alcohol (PVA), polycarbonate (PC), polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET) and polyether sulfon (PES), but it is almost impossible to decrease the viewing angle dependence in practical use with such films.
The viewing angle dependence of the compensation plate, can be improved when a refractive index in a direction perpendicular to a plane of the plate is made larger than the minimum value among refractive indexes with the plate [refer to Japan Display, 1989, p. 339 and Japanese Patent Application S.N. 30019 (1989)]. However, when a film made of one of general polymers as described above is stretched, there is generally a relationship: EQU n.sub.x &gt;n.sub.y &gt;n.sub.z or n.sub.x &gt;n.sub.y .div.n.sub.z
wherein n.sub.x is a refractive indexes in a stretching direction x, n.sub.y is a refractive index in a direction y perpendicular to the direction x in the plain and n.sub.z is a refractive index in a direction perpendicular to the plane.
Structure of the general polymers described above are represented by the formulae: ##STR1##
In the polymers having such chemical structures, it is understood that a refractive index is large in a direction of their main chains and the main chains are aligned in the direction of stretching, x, consequently, n.sub.x becomes the largest among the three. Further, in general monoaxial stretching, it is also considered that the polymer is stretched only slightly or not stretched substantially in the direction y, while in the direction z, it somewhat shrinks along the direction of thickness. Accordingly, the relation of the three indexes is; EQU n.sub.x &gt;n.sub.y &gt;n.sub.z or n.sub.x &gt;n.sub.y .div.n.sub.z
Although an optical property of the compensation plate can be enhanced by improving the stretching method, it extremely difficult to change an order of the refractive indexes if a chemical structure of the polymer used is similar to that of the general polymer described above.
In the reference "Japan Display" cited before, a cell having liquid crystal molecules aligned perpendicular to a plane of the substrate is used as a compensation plate in which the refractive index in the direction perpendicular to the plane is made greater than the minimum refractive index within the plane.
However, this involves disadvantages, such as, an increased thickness of the device and difficulty to prepare such compensation plate.
The present inventors have made an earnest study for overcoming the foregoing disadvantages in the conventional liquid crystal display devices and obtaining a compensation plate which can provide an excellent effect of viewing angle compensation to a display device responding on their mode and which is easy to manufacture, and as a result, have found that a compensation plate in which a refractive index in a direction perpendicular to a plane of the plate is greater than the minimum refractive index in the plane can be obtained easily, by making at least one layer of the compensation plate out of a polymer film in which multiple bonds are present in its side chain and a number of atoms bonded between the main chain and the multiple bonds is in a range of 0 to 5, or a polymer film comprising a liquid crystalline polymer, of which liquid crystal group is aligned perpendicular to the plane and have accomplished the present invention based on the findings.